Electrospray ionization (ESI) is a method of generating ions in the gas phase at relatively high pressure. ESI was first proposed as a source of ions for mass analysis by Dole et al. (Dole, M.; Mach, L. L.; Hines, R. L.; Mobley, R. C.; Ferguson, L. P.; Alice, M. B. J. Chem. Phys. 1968, 49, 2240-2249). The work of Fenn and coworkers (Yamashita, M.; Fenn, J. D. J. Phys. Chem. 1984, 88, 4451-4459; Yamashita, M.; Fenn, J. D. J. Phys. Chem. 1984, 88, 4671-4675; Whitehouse, C. M.; Dreyer, R. N.; Yamashita, M.; Fenn, J. B. Anal. Chem. 1985, 57, 675-679) helped to demonstrate its potential for mass spectrometry. Since then, ESI has become one of the most commonly used types of ionization techniques due to its versatility, ease of use, and effectiveness for large biomolecules.
ESI involves passing a liquid sample through a capillary which is maintained at a high electric potential. Droplets from the liquid sample become charged and an electrophoretic type of charge separation occurs. In positive ion mode ESI, positive ions migrate downstream towards the meniscus of a droplet which forms at the tip of a capillary. Negative ions are attracted towards the capillary and this results in charge enrichment in the growing droplet. Subsequent fissions or evaporation of the charged droplet result in the formation of single solvated gas phase ions (Kebarle, P.; Tang, L. Analytical Chemistry, 1993, 65, 972A-986A). These ions are then usually transmitted to a downstream aperture of an analysis device such as a quadrupole mass spectrometer, a time of flight mass spectrometer, an ion trap mass spectrometer, an ion cyclotron resonance mass analyzer or the like.
Ionspray is a form of ESI in which a nebulizer gas flow is used to promote an increase in droplet fission. The nebulizer gas aids in the break-up of droplets formed at the capillary tip. Ions formed in this manner can be directed into the vacuum system of various mass analyzers which include, but are not limited to, quadrupoles, time of flight, ion traps and ion cyclotron resonance mass analyzers.
Unfortunately, the use of ESI and ionspray with mass spectrometers results in poor ion sampling efficiency. Typically, the majority of ion losses occur between the atmospheric pressure region, where the ions are generated, and the first differentially pumped vacuum stage that the ions must enter. Ions are formed in a broad plume of the electrospray, typically up to 1 cm in diameter. The ion sampling orifice, i.e. inlet orifice of the mass spectrometer, is typically about 0.01 to 0.025 cm in diameter, and so only a small fraction of the ions pass through the sampling aperture. The size of the aperture separating the atmospheric pressure region from the first vacuum stage provides a conductance limit for the flow of gas and ions into the mass spectrometer. The diameter of the aperture is limited by the pumping speed of the vacuum system of the mass spectrometer. Due to the substantial expense associated with vacuum pumps, a compromise must be reached between the desired aperture size and the cost of the vacuum pumps. In addition, since the ion motion at atmospheric pressure is dependent upon the shape and distribution of the equipotential lines, many ions are not directed to the inlet aperture.
Accordingly, there have been attempts to increase the ion sampling efficiency which have led to the development of nanoelectrospray ionization (Wilm, M.; Mann, M. Anal. Chem. 1996, 68, 1-8) and other reduced flow rate electrospray ionization sources (Figeys, D.; Aebersold, R. Electrophoresis, 18, 1997, 360-368). Reduced flow-rate ionization sources make use of a tapered sprayer with an internal diameter that is much smaller than those used in typical ESI sources. Reduced flow rate ion sources typically have a flow rate of 0.05 to 1.0 μL/min and have a tapered sprayer with an internal diameter of 5-30 μm. Typical ESI and ionspray sources have flow rates of 1-1000 μL/min and sprayer tip diameters of 50-200 μm. For a given analyte concentration, the signal with a reduced flow-rate ion source is typically as great as or greater than that of conventional electrospray sources even though much lower flow rates are required. This is a result of the substantial increase in the sampling efficiency of the analyte ions generated by the source. Reduced flow-rate ion sources may also incorporate a nebulizer gas flow. These types of ion sources are referred to as reduced flow-rate ionspray sources in the text that follows.
Another approach that can be used to increase the ion sampling efficiency of ESI for mass spectrometry involves modifying the mass spectrometer to which the ESI source is attached. In particular, the diameter of the entrance aperture of the mass spectrometer may be increased in order to draw more ions into the vacuum system. Provided that the ion to gas ratio remains constant, an increase in the ion signal is expected to be proportional to the increase in the gas flow. However, a larger vacuum pump will be required to maintain the same pressure within the mass spectrometer. Unfortunately, increasing the vacuum pump speed results in a mass spectrometer with a substantially higher cost.
Prior art methods have looked at applying potentials in a vacuum region or regions or a transition region or regions which are at reduced pressures to reduce the spread of the ions, i.e. to focus the ion beam. However, this is difficult because the ion spread is controlled by both equipotentials and gas velocity within the reduced pressure region or regions. Also, if an inappropriate potential were applied to the lens elements, undesirable ion fragmentation may result. Conversely, in an atmospheric pressure region, it is the equipotentials which dominate the ion trajectories and the distance that the ions travel between collisions is so short that the ions do not accumulate enough energy to effect ion fragmentation or to achieve significant velocity.
Ion lenses have been used in vacuum regions to focus ion beams and alter ion trajectories. Other prior art methods are directed towards improving ion trajectories immediately prior to entry into a downstream mass spectrometer. Franzen et al. (U.S. Pat. No. 5,747,799) described a ring electrode positioned on the inside wall of a heated capillary inlet, which was at or near atmospheric pressure, for a mass spectrometer that was downstream of an ESI source. The ring was intended to help draw ions into the inlet capillary of the mass spectrometer. The ring improved the shape of the equipotentials such that the electric field lines were pointed directly into the inlet capillary of the mass spectrometer. However, no evidence was given as to whether an appreciable increase in the ion signal was observed.
Gulcicek et al. (U.S. Pat. No. 5,432,343) disclosed an interface for an ESI source, at atmospheric pressure, connected to a mass spectrometer that contained a transition region with multiple vacuum stages. The transition region included at least one electrostatic lens that had to be properly positioned to aid in focusing the ions along a centerline. The electrostatic lens was intended to increase the ion transmission efficiency through the second and third differentially pumped stages of vacuum. In the ESI source housing, Gulcicek showed an end plate lens element and a cylindrical lens which was placed near the perimeter of the housing of the ESI source. The lenses in the ESI source housing were intended to help enrich the concentration of charged droplets near the centerline, in the ESI source, where the desorbed analyte ions could be more efficiently swept into a capillary entrance which led to the transition region. However, these lenses were located at a substantial distance from both the sprayer and the inlet aperture of the capillary that led to the transition region so it is questionable as to how much of a focusing effect the lenses in the source housing provided near the sprayer tip. While details of electric fields are given for other parts of the apparatus, no details are given of the electric field in this atmospheric ionization chamber. Furthermore, no results were shown to indicate that an increase in ion signal is achievable with this method.
Feng et al. (Feng, X.; Agnes, G. R. J. Am. Soc. Mass. Spectrom. 2000, 11, 393-399) evaluated several atmospheric pressure electrode designs to guide ions into the sampling orifice of a downstream mass spectrometer. The wire lenses were located downfield from a droplet levitation ion source. The flow rate of the ion source was 5 μL/min. Feng et al. found that the wire lenses led to increased ion currents detected within a mass spectrometer. However, the lenses used both AC and DC voltages which requires a more expensive power supply. Furthermore, the Feng device cannot be used with a curtain gas, therefore the practical use is limited. In addition, the Feng lens has been demonstrated to work only with single isolated droplets and not with a continuous ion source like an ESI source. Finally, the Feng lens is located in the desolvation region substantially downfield from the source of ions.
Whitehouse et al. (U.S. Pat. No. 6,060,705) added windows along an atmospheric pressure ionization chamber to allow for direct viewing of the electrospray and the atmospheric pressure ion source during operation. Whitehouse also disclosed a cylindrical electrode extending along the side walls of the atmospheric pressure ionization chamber and a nebulizer gas flow which was applied to the electrospray needle tip. There were also three electrostatic lenses in a transition region between the ion source and a downstream mass spectrometer. The potential of the cylindrical electrode within the source housing was set so that the charged ions which left the electrospray needle tip were directed and focused by an electric field towards an orifice or capillary entrance of the downstream mass spectrometer. Whitehouse noted that there was an increase in the ion signal when the potential applied to the cylindrical electrode, within the source housing, was increased, as well as when a potential was applied to the cylindrical lens and a nebulizer gas was used to aid in breaking-up the charged droplets. Whitehouse also demonstrated that the potentials and the needle position could be adjusted to optimize the electrospray performance. However, once again, the cylindrical electrode within the ESI source housing was far away from the ESI sprayer. Furthermore, the configuration of the cylindrical electrode was fixed, and the position or orientation of the electrode could not be adjusted.
Bertsch et al. (U.S. Pat. No. 5,838,003) disclosed an electrospray ionization chamber which operated substantially at or near atmospheric pressure and incorporated an asymmetric electrode. The asymmetric electrode was either one half of a full cylinder, a flat semicircular plate, a wire or a flat circular disk. The sprayer was oriented at a 90 degree angle to the axis of the ion entrance of the mass spectrometer. Bertsch also disclosed that the electrode may have extended past the tip of the sprayer. However, Bertsch demonstrated that the asymmetric electrode was required to initiate and sustain the electrospray. It appears that the asymmetric electrode is maintained at the same potential as a counter electrode, i.e. similar to other prior proposals there is no clear teaching of a separate lens maintained at a potential different from that of two electrodes establishing the basic electric field. Bertsch also taught that their device was applicable for flow rates of 1 μL/min up to 2 ml/min and thus was not applicable for reduced flow-rate ESI sources. Bertsch also stated that a nebulizer gas may be introduced to assist in the formation of an aerosol.
In other work, Tang et al. (Tang, K.; Lin, Y.; Matson, D.; Taeman, K.; Smith, R. D. Anal. Chem. 2001, 73, 1658-1663) disclosed multiple microelectrospray emitters which successfully generated stable multielectrosprays in a liquid flow rate range (1 to 8 μL/min total flow) compatible with mass spectrometry. Higher total electrospray ion currents were observed as the number of electrosprays increased at a given total liquid flow rate. Tang also disclosed that stable electrosprays could be generated at higher liquid flow rates compared to conventional single ESI sources in which the electrospray was generated from a fused-silica capillary. A nebulization gas may also be used with the multiple microelectrospray emitters.
In light of the prior art, a need still remains for an inexpensive apparatus that can be used to focus ions, as they are generated at the capillary tip, to increase the ion flux into a downstream device such as a mass spectrometer. It is especially important to note that very few studies to date have focused on methods of improving ion trajectories as the ions are generated in the sprayer plume of an ion source.